1. Field of the Invention
The present invention relates to a permanent magnet rotor (hereinafter, briefly referred to as rotor) for a synchronous electric motor.
2. Description of the Related Art
As a rotor of a synchronous electric motor, commonly, either a radial type rotor or a surface magnet type rotor is used. The radial type rotor is configured by sequentially disposing a rotor core forming a yoke and a permanent magnet in an alternate manner in a circumferential direction. On the other hand, the surface magnet type rotor is configured by sequentially sticking a plurality of permanent magnets on an outer circumferential surface of a cylindrical rotor core in a circumferential direction.
On the permanent magnets of the rotor as described above, a centrifugal force following rotation of the rotor and a magnetic attraction force due to a magnetic field of a stator disposed around the rotor act at a radial direction outer side of the rotor. Consequently, particularly in the surface magnet type rotor, the permanent magnets must be fixed so that even when the centrifugal force and the magnetic attraction force as described above act on the permanent magnets for a long period, the permanent magnets are not separated from the outer circumferential surface of the rotor core.
Accordingly, in the surface magnet type rotor, hitherto, permanent magnet fixing methods as illustrated in FIG. 6-8 have been proposed. FIGS. 6-8 are diagrams illustrating an enlarged cross-sectional structure of a part of a rotor according to conventional methods 1-3, respectively. In particular, the diagrams are views in which a permanent magnet fixing structure relative to a rotor core is seen from a direction along a rotation axis of the rotor.
In the conventional method 1 as illustrated in FIG. 6, each of a plurality of permanent magnets 101 is bonded and fixed to an outer circumferential surface of a rotor core 103 by a resin or a bonding agent 102.
In the conventional method 2 as illustrated in FIG. 7, while each of the plurality of permanent magnets 101 is sequentially disposed on the outer circumferential surface of the rotor core 103, an insulation tape 104, such as a glass cloth, is wound around an outer circumference of the rotor core 103. Thereby, each permanent magnet 101 is sandwiched between the rotor core 103 and the insulation tape 104 to be fixed.
In the conventional method 3 as illustrated in FIG. 8, both side surfaces of the permanent magnets 101 are formed into a tapered shape and a plurality of engagement portions 105 having a reverse taper shape that tightly engage with the both side surfaces of the permanent magnets 101 are sequentially provided on the outer circumferential surface of the rotor core 103, thereby fixing each permanent magnet 101 using a wedge effect.
In addition, a synchronous electric motor including such a rotor as described above is often adopted for a feed axis of a machine tool. Since the smoothness of rotation of the electric motor greatly influences a machining accuracy of the machine tool, designing the rotor such that a cogging force of the electric motor is as small as possible has been sought.
To reduce the cogging force as described above in the surface magnet type rotor, forming an adequate curved surface on a surface of the permanent magnets opposed to an inner circumferential surface of the stator is needed.
Generally, in the case of the permanent magnets designed as described above, as disclosed in Japanese Laid-open Patent Publication No. H09-205747, Domestic Re-publication of PCT International Application No. 2006-008964, and Japanese Laid-open Patent Publication No. 2015-122842, for example, a thickness size becomes smaller as from a center portion to an end portion of the permanent magnets in the circumferential direction of the rotor core (unillustrated in FIGS. 6-8).
Further, Japanese Laid-open Patent Publication No. H09-205747, Domestic Re-publication of PCT International Application No. 2006-008964, and Japanese Laid-open Patent Publication No. 2015-122842, for example, disclose a plurality of projection portions (unillustrated in FIGS. 6-8) disposed in such a manner as to sandwich each permanent magnet and projecting from the rotor core to a radial direction outer side of the rotor core. Then, the projection portions have a shape for tightly engaging with the end portion of the permanent magnets in the circumferential direction of the rotor core so that the permanent magnets are not separated from the rotor core.
However, when the thickness size of the permanent magnets is configured to become smaller as from the center portion to the end portion of the permanent magnets in the circumferential direction of the rotor core as described above, a permeance coefficient and a demagnetization resistant capacity are more reduced as closer to the end portion of the permanent magnets. As a result, there occurs a problem in that, also when the same demagnetizing field is applied to the permanent magnets in the circumferential direction of the rotor core, the end portion of the permanent magnets is more apt to be demagnetized than the center portion of the permanent magnets.
In addition, when the projection portions are configured to have a shape for tightly engaging with the end portion of the permanent magnets as described above, the permeance coefficient and the demagnetization resistant capacity of the end portion of the permanent magnets are remarkably increased, which accordingly makes it difficult to reduce the cogging force.
Thus, in the surface magnet type rotor, realizing reducing the cogging force and restraining a reduction of the demagnetization resistant capacity of the end portion of the permanent magnets in a balanced manner has been desired.